The use of the so-called six-gate technology in the field of optical transmission technology, e.g. with coherent optical receivers, is known, for example, from the IEEE Publication “Multi-Port Technology for Microwave and Optical Communications” by I. Molina-Fernández et. al. According to the aforementioned IEEE Publication, an electrical signal with unknown frequency is transmitted to a six-gate circuit via a delay line. As a result of the delay line, the two signals are received with a phase difference at the six-gate circuit, which difference is proportional to the unknown frequency. This phase difference is detected by the six-gate circuit, so that the unknown frequency can be computed with the aid of the length for the delay line.